This invention relates to the field of automatic frequency control and, more particularly, to the provision of a simple and inexpensive means of automatic frequency control for the local oscillator used with an AM stereophonic decoder.
In a manually tuned receiver used for decoding AM stereo signals, the oscillator in the decoder is typically of a type which is more easily "pulled" into a locked condition than a crystal oscillator would be. This means that the decoder oscillator is not as sharply tuned as is desirable and that the gain of the local oscillator loop must be kept as constant as possible over the tuning range of the receiver. This requirement is due to the fact that, as the gain decreases, the corner frequency of the pilot tone detector also decreases, causing an increase in the output of the pilot tone bandpass filter if there is any input, pilot tone or otherwise. If severe enough, this increase in gain can cause "falsing" or activation of the stereo mode indicator and the stereo mode switching circuit, particularly at the ends of the tuning range. More important, perhaps, is the effect of gain change in the local oscillator AFC loop on the pull-in range. If the range were reduced far below the desired range of .+-.7 kHz, for example, it would be necessary to tune the receiver very accurately in order to achieve lock-in. On the other hand, if the range were much greater than .+-.7 kHz, it could be difficult to separate stations, or the receiver might lock in on one station and stay locked until the next one had been passed by. Both of these effects are highly undesirable, even in an inexpensive stereo receiver, yet any solution must obviously be inexpensive.